I've actually heard that at first, they didn't even want players to be able to return to the prison (and kill all the guards, thus forcing more reactions they'd have to add).
I think it's not a terrible idea to gate off areas you've already finished and prevent you from returning there - as long as it's made extremely clear that it is a one-way trip so you better finish everything first. In my view it can be better for immersion even if it can make some things less convenient.
